The lifespan of a marble shelf can vary significantly based on several factors:

1. **Quality of Marble:** High-quality marble, properly cared for, can last for decades or even longer. Lower-quality marble or marble with structural weaknesses may have a shorter lifespan.

2. **Maintenance:** Regular cleaning, proper sealing (if applicable), and maintenance routines can significantly extend the lifespan of a marble shelf. Neglecting maintenance can lead to deterioration over time.

3. **Usage and Weight Bearing:** How the shelf is used impacts its longevity. A marble shelf used for heavy items or subjected to frequent stress may experience wear and tear more quickly.

4. **Environmental Factors:** Exposure to moisture, extreme temperatures, or acidic substances can damage marble. Protecting the shelf from these elements can prolong its lifespan.

5. **Installation Quality:** Proper installation, including correct mounting and support, ensures the shelf is structurally sound. Poor installation can lead to stress points and reduce the shelf's durability.

6. **Handling and Care:** Careful handling during installation and throughout its lifespan is crucial. Avoiding impacts or scratches can preserve the shelf's integrity.

Generally, with proper care and maintenance, a well-built and adequately supported marble shelf can last several years to several decades. Regular cleaning, using appropriate cleaning products, and addressing any damage promptly can help maximize its lifespan.